Spermatozoa from male mice heterozygous for abnormal alleles of the T locus are different in their physiology, morphology, and antigenic surface components than spermatozoa from normal males. The effect of these differences upon their function in fertilization is not known. I propose to observe the timing and morphology of sperm penetration and formation of pronuclei in eggs of normal females mated to males heterozygous for tw32. I also intend to set up a fertilization in vitro system with these gametes, so that continuous observations of the same zygote may be possible. Consistent differences from the congenic control males (not carrying tw32) in these early stages in fertilization may indicate how the abnormalities associated with this t allele relate to sperm function. This knowledge in turn may be useful in uncovering physiological or structural mechanisms as yet unknown, but necessary for normal sperm penetration or formation of pronuclei in mammalian fertilization.